1. Field of the Invention
The present invention relates to monolithically integrated two-dimensional image sensors with sensor elements arranged in rows and columns on a semiconductor substrate and in particular to such an image sensor with a difference-forming stage for undertaking sequential readouts of the sensor elements to generate a substantially noise-free signal.
2. Description of the Prior Art
A monolithically integrated two-dimensional image sensor in which the sensor elements are arranged in rows and columns on a doped semiconductor body is known from German AS No. 26 11 711. Each sensor element in a respective column is connected by a common column line which represents a readout line for the sensor signals from the elements in that column. Each column line is connected to an input of a readout device which has a number of outputs for sequential readout of the sensor signals. The image sensor further has a difference-forming stage for determining the difference between signals representing incident radiation plus noise and signals representing substantially noise alone so that a final signal can be generated which corresponds only to charge carriers generated by radiation incident on the sensor elements. The difference-forming stage known from German AS No. 26 11 711 undertakes a double sampling of the voltage signals and requires a relatively large surface area on the semiconductor substrate and can only be co-integrated on the substrate with other circuit components with substantial difficulty.
Another circuit for undertaking a difference-formation of electrical charges integrated on a semiconductor body is known from German OS No. 28 11 146 in which the charges to be measured are successively transferred in a charge coupled arrangement into a capacitor formed by an electrode with a freely adjustable potential disposed above the semiconductor body, and the charges are removed therefrom with the corresponding potential difference being measured at the electrode. Employment of this circuit as a readout means for a two-dimensional image sensor would require simultaneous transfer of the total sensor signals and the noise signals which would also require a substantial surface area outlay on the semiconductor body.
Sensor elements of the type which may be used in the invention disclosed herein are known, for example, from the text by P. G. Jespers entitled "Solid State Imaging", Noordhof Int. Publishing, Leyden, The Netherlands at pages 447 through 461 as well as a publication by S. Ohba entitled "A 1,024 Element Linear CCD and Sensor With a New Photodiode Structure", Proceedings IEDM 1977, Washington, at pages 538 through 541.
The basic structure and operation of all types of charge transfer device arrangements which may be used within the context of the inventive concept disclosed herein are known, for example, from the text by Sequin and Tompsett entitled "Charge Transfer Devices", Academic Press, New York, 1975 at pages 1 through 18.